问题补充:
He is one of the world-famous musicians. By the time he was six, he composing.
A. had begun
B. begins
C. would begin
D. was beginning
答案:
(答案→)A. had begun
B. begins
C. would begin
D. was beginning
解析:考查谓语动词的时态 by +过去时间,谓语动词(主句)常用过去完成时态;by+将来时间,谓语动词(主句)常用将来完成时态